1. Field of the Invention
The present invention relates to the control of feedback in a servo control system. More particularly, the invention relates to the control of mechanical feedback from vibration in an optical disk drive to prevent destabilization of a servo control system in the optical disk drive.
2. Discussion of the Related Art
Modern computers include high performance main memory used to store user data and the instructions for processing the user data. Main memory is generally in the form of solid state microchips which operate at rapid, purely electronic speeds. Because main memory is also very expensive, it is usually supplemented with peripheral storage devices. Such devices include, for example, magnetic tape drives, magnetic disk drives, and optical disk drives. As used herein, the term "peripheral storage device" also includes similar devices used outside of the data processing environment, such as those used for audio and/or video recording.
Peripheral storage devices typically include one or more servo control systems (hereinafter "servos"). A servo is an automatic control system in which an output is constantly or intermittently compared with an input so that the error or difference therebetween can be used to bring about the desired operation. In peripheral storage devices, the output is typically an operating characteristic and is used to adjust the operation of the device directly relating thereto. For example, in magnetic disk drives, a tracking servo may be used to control the amount of physical misalignment, if any, between a data track on the magnetic disk and the read/write elements of the magnetic disk head. The misalignment may be caused by radial runout of the tracks on the disk, which is one or more radial imperfections resulting from manufacturing tolerances or the operating characteristics relating thereto. As the misalignment is monitored, the relative position of the disk to the head is continually adjusted/corrected to allow for the proper reading/writing of data to a desired track. In optical disk drives, a focus servo is also used to accommodate the axial runout (similar to radial imperfections, but axial in nature, such as variations in disk thickness) of a disk. This servo monitors the distance between an optical disk and the objective lens of the optical disk drive. The objective lens has a fixed focal length; the focus servo is used to adjust the relative position of the objective lens to the disk to maintain the light beam of the optical disk drive in proper focus upon the active recording layer of the disk. Techniques for monitoring track misalignment and the distance between an optical disk and an objective lens are well known.
Many servos are used to control mechanical actions, as in the previously-described servo examples. Using the focus servo of an optical disk drive as a further example, the mechanical action is the activation of a voice coil motor or other actuator to move the objective lens closer to or farther from the surface of the optical disk. When no feedback is present, there is no particular phase or amplitude relationship between the vibration of the optical disk and the electrical input to the actuator. However, depending upon the specific physical configuration of a device, the mechanical action can result in certain additional, generally undesirable movement which is fed back into the servo. Again using the focus servo as an example, the action of the motor to adjust the position of the objective lens may result in vibration being passed through the actuator mounting, through the base or housing of the optical disk drive, through the spindle motor, and to the optical disk. As the optical disk vibrates, the distance between it and the objective lens varies further. Such additional vibration of the disk due to mechanical feedback, and not from axial runout, effects the operation of the servo according to certain characteristics of the feedback. The magnitude of the mechanical feedback becomes very large at the resonant frequency of the optical disk and, if the resonant frequency of the disk is close to the crossover frequency of the servo (the crossover frequency is that at which the open loop servo response crosses through zero dB magnitude), the phase margin at such crossover frequency will be degraded. If so, the amplitude of the disk motion resulting therefrom increases dramatically and destabilizes the servo loop.
Destabilizing mechanical feedback is typically controlled by separation of the feedback source or by reduction of the feedback amplitude. Feedback control by separation does not refer to physical separation (which is assumed not possible here), but to separating the resonant frequency and the crossover frequency. Again, using the focus servo example, the resonant frequency of the disk relative to the crossover frequency of the servo could be adjusted by altering the mechanical or operating characteristics of either. At one time, such changes could be made with relative ease, but heretofore unrecognized problems now exist. Because of the now common standardization of peripheral recording media, there is little room to change the resonant frequencies thereof if a device is to be commercially viable. In addition, it is not desirable to adjust the crossover frequency of an actuator lower, as such reduces the performance of the servo. Finally, in modern finely tuned actuators, it is difficult to increase the crossover frequency of an actuator without significantly increasing costs.
Feedback control by amplitude reduction is accomplished by simple stiffening of the device components in either of two ways. First, the materials used to manufacture the components may be selected for their natural mechanical properties. For example, steel may be used instead of aluminum or flexible plastics. Second, regardless of the materials used, the components can be thickened or otherwise made more massive to increase their stiffness and decrease the likelihood of their vibrating. Unfortunately, simple stiffening of the device components is also associated with several heretofore unrecognized problems. First, peripheral storage devices are already manufactured from relatively stiff materials to provide adequate sturdiness. Further materials improvements require more exotic, and expensive, materials. The use of thicker components also adds to the expense of the device by requiring more of each material used. In addition, thicker components add to the weight of the device, reducing transportability. Finally, thicker components are not possible in certain space limited environments, such as peripheral storage devices mounted internally in personal computers and workstations, or those mounted in standard size racks for use in larger data processing systems.